1. Field of the Invention
The present invention relates to a method and apparatus for disposal of sewage sludge. In particular, it relates to a method and apparatus for drying and combustion of sludge in connection with refuse incineration.
2. Discussion of the Related Art
Various methods have been developed over the years to dispose of sewage sludge, the discharge from waste water purification. Such methods include landfilling, composting, wet spreading and drying for fertilizer. However, these methods are proving to be controversial and unacceptable in today""s environment. Also, landfills have become increasingly expensive as landfills are closed and transportation costs have increased to new locations. Landspreading is not sufficiently sanitary, resulting in leaching of some sludge byproducts into fresh water supply systems. Although sewage sludge is rich in nutrients, its heavy metals, pathogens and other chemical contents have raised questions about its acceptability for spreading on farmland, in either the wet or dry forms. Also, the drying of sewage sludge for fertilizer requires significant energy, which has become more expensive.
More recently, incineration has developed as a way to dispose of sewage sludge and to reduce reliance on other fuel sources. Since sewage sludge has a high water content, as much as 95%, dewatering and drying processes are required prior to incineration. Typically, dewatering is carried out with filter presses, belt filter presses or centrifuges. Drying uses cylindrical rotary kilns, kilns with overlying beds, fluidized bed furnaces, grinding driers or indirect driers of different constructions. A representative apparatus for incineration of sewage sludge is disclosed in U.S. Pat. No 3,954,069. Another such apparatus is disclosed in U.S. Pat. No. 4,311,103. The steps of dewatering and drying require significant energy and transport of the sewage sludge, with different water contents, to various locations for these processes to occur. These systems also require a complicated infrastructure with multiple pieces of interconnected processing equipment.
In order to address disposal of both sewage sludge and household or municipal solid waste, different waste combustion systems have been developed. Initial systems attempted to mix the sewage sludge and municipal solid waste together prior to being fed to the waste to energy boiler. However, in such a system, the overall water content of the sludge and waste results in poor combustion and slower feed rates. To overcome this problem, one system, disclosed in U.S. Pat. No. 4,753,181, fed the sewage sludge separately from the municipal waste. In this system, the sewage sludge is dispersed in small droplets over the flame in front of the combustion zone. The droplets are dried and incinerated by the hot air in the furnace. However, since the sludge is still in a liquid state, the flow rate must be controlled to keep the moisture content in the furnace within acceptable levels.
U.S. Pat. No. 5,410,973 discloses a system which uses the flue gases from refuse combustion and sewage sludge incineration in processing the other type of waste. The sewage sludge is incinerated in a rotary tubular kiln. Flue gases from the refuse furnace are drawn in the opposite direction of sludge flow for drying and incinerating the sludge in a single process stage. The flue gases from the kiln are fed back into the refuse furnace for afterburning. This system, however, requires two distinct furnaces, one for sludge and one for refuse. The materials cannot be combined. Additionally, when the flue gasses are transferred between the furnaces, they must be kept hot enough so as not to adversely affect the incineration process.
U.S. Pat. No. 5,630,366 discloses another system in which the sewage sludge is dried before it passes to the incinerator. The sludge is dehydrated by centrifuging-drying using the heat of the fumes from the refuse furnace. The dried sludge is then combined with the municipal waste to be incinerated. This system makes use of the hot gases from the furnace in drying the sludge, but requires transportation of sludge in various forms to different locations. Furthermore, the gases must be directed to the drying chamber for the sludge.
Therefore, a need exists for a simple system for processing sewage sludge for incineration. A need exists for a system which can incinerate sewage sludge in conjunction with municipal waste. A need exists for a system which can dry sewage sludge before combustion without significant energy uses and without requiring transport to multiple processing areas.
The present invention overcomes deficiencies of prior art systems by providing a single processing stage for transporting and drying the sewage sludge, the dried sludge can be directly injected into a typical municipal waste to energy plant incorporating the invention. The present invention includes a drying system having a spiral feed screw without a central shaft. Heated air, which could be from a furnace, is directed through the center of the drying system for drying the sewage sludge. A high energy inductor at the end of the feed system provides the draw for the heated air. The high energy inductor also expels aspirated, dried sludge. When incorporated into a waste to energy plant, the drying system can feed the dried sludge directly into the combustion zone of the furnace, resulting in combustion of the sludge. The municipal waste to energy plant can be operated in an ordinary manner without the need for additional energy or processing.